The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic imaging process as disclosed in U.S. Pat. No. 2,297,691 describes placing a uniform electrostatic charge on a photoconductive insulating layer known as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic toner material. The toner will normally be attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper. The transferred image subsequently may be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or over coating treatment.
Electrostatic images formed on an electrophotographic photoconductor and an electrostatic recording medium are typically developed by using (i) a mono-component type dry developer consisting of a toner including a coloring agent such as a dye or pigment and a binder resin in which the coloring agent is dispersed, or with addition of a charge controlling agent thereto when necessary, or (ii) a two-component type dry developer including the above-mentioned powder-coated toner and solid carrier particles. Toners and developer compositions including colored particles are well known. In this regard, see U.S. Pat. Nos. 5,352,521; 4,778,742; 5,470,687; 5,500,321; 5,102,761; 4,645,727; 5,437,953; 5,296,325 and 5,200,290, the disclosures of which are hereby incorporated in their entirety by reference. A traditional toner composition typically contains toner particles consisting of a binder resin and colorants, a wax or a polyolefin, a charge control agent, flow agents and other additives. A typical toner formulation typically contains about 90–95 weight percent resin, about 2–10 weight percent colorant, about 0–6 weight percent wax, about 0–3 weight percent charge control agent, about 0.25–1 weight percent flow agent and 0–about 1 weight percent other additives. Widely-used binder's resins are styrene-acrylic copolymers, styrene-butadiene copolymers and polyesters. The colorants usually are selected from cyan dyes or pigments, magenta dyes or pigments, yellow dyes or pigments, and mixtures thereof.
In the course of an electrophotographic printing operation, toner particles are subjected to a number of complex steps such as charging, electrostatic transfer, fusing, etc. There is now a realization that toner particles must possess a balance of compositional, geometrical and physical properties to perform well in a modern electrographic printer. Examples of such properties are a small mean particle size, a uniform size distribution, fast and stable electrostatic charging, fast melting, good particle flowing characteristics, controlled presence of internal additives, such as wax and charge control agent.
For example, it is difficult to obtain resolutions better than about 600 dots/inch when the average particle size is larger than about 7 μm. For resolutions, in the order of about 1200 dots/inch, particle sizes smaller than 5 μm are typically needed. Conventionally, color toner particles are produced by a mechanical milling process, for example, described in the U.S. Pat. No. 5,102,761. In that process, an acrylate resin is compounded with a pigment, a charge control agent (“CCA”), and occasionally a wax in a melt mixer. The resulting polymer mixture is mechanically crushed and then milled into small particles. Such a conventional toner process typically produces particles with an irregular shape and a broad distribution in particle size.
Improvements in the methods of producing small toner particles with a uniform size distribution have been attempted in the past. For example, the aforementioned U.S. Pat. Nos. 5,352,521, 5,470,687 and 5,500,321 disclose toner particles produced by dispersion polymerization. In such a method, monomers (typically styrenic and acrylate monomers) and additives such as pigment, charge control agent and wax are mixed together to form a monomer dispersion. This is then further dispersed into an aqueous or a non-aqueous medium and the monomer is reacted to form toner particles. This method has the advantage over the aforementioned milling methods that spherical toner particles with a small mean diameter can be prepared. However, the polymerization involves a substantial volume contraction and it results in entrapment of the dispersion medium inside the resulting particles. Furthermore, the polymerization is difficult to complete and a substantial portion of the monomers remain in the toner particles. The residual monomers and the entrapped dispersion solvent are difficult to remove from the particles. Further, agents employed, such as dispersion-stabilizing agent and surface active agent, cause the charging characteristics and preservability of the toner particles to deteriorate by remaining on the surface of the toner particles and are difficult to remove.
Another so-called chemical method for forming small toner particles is the emulsion aggregation method disclosed in U.S. Pat. Nos. 5,916,735 and 6,268,103. In a typical emulsion aggregation method, emulsion particles of sub-micron size are first formed using an emulsion polymerization process and toner particles are produced by aggregating the emulsion particles and subsequent drying. The method consists of many delicate process steps including the aggregation step and an extensive drying step.
Additionally, the above-described chemical methods of manufacturing toner particles are applicable only for resins such as styrenic copolymer resins that are polymerized by an addition reaction. The methods therefore cannot be used for a polyester resin which is polymerized by a condensation reaction.
U.S. Pat. No. 6,132,919 discloses a high-resolution toner composition with a core-shell structure, prepared by a suspension polymerization process. In this case, core particles are first prepared by polymerizing a suspension of monomer for core resin containing a colorant and toner particles and toner particles are then prepared by polymerizing shell monomer on the surface of the core particles that are dispersed in a liquid medium. There are several important limitations with such a core-shell toner. First, the core resin is required to have a lower glass transition temperature than that of the shell resin. The amount of colorant that one can incorporate is limited. Lastly, the process is impractically complex.
There is continuing interest in developing improved toner composition including particles which have a novel arrangement of the component compounds and enabling methods of producing the toner particles with such a unique structure and properties for high-resolution color electrophotography.